1. Field of the Invention
This invention is directed to a magnesium electrochemical cell containing as an active cathode material of MnO2 nanoparticles which have a surface area greater than 60 m2/g and which provides high cell capacity and increased cycle lifetime. The invention is further directed to a magnesium battery containing a cathode having the MnO2 according to the invention as an active ingredient.
2. Discussion of the Background
Lithium ion batteries have been in commercial use since 1991 and have been conventionally used as power sources for portable electronic devices. The technology associated with the construction and composition of the lithium ion battery (LIB) has been the subject of investigation and improvement and has matured to an extent where a state of art LIB battery is reported to have up to 700 Wh/L of energy density. However, even the most advanced LIB technology is not considered to be viable as a power source capable to meet the demands for a commercial electric vehicle (EV) in the future. For example, for a 300 mile range EV to have a power train equivalent to current conventional internal combustion engine vehicles, an EV battery pack having an energy density of approximately 2000 Wh/L is required. As this energy density is close to the theoretical limit of a lithium ion active material, technologies which can offer battery systems of higher energy density are under investigation.
Magnesium as a multivalent ion is an attractive alternate electrode material to lithium, which can potentially provide very high volumetric energy density. It has a highly negative standard potential of −2.375V vs. RHE, a low equivalent weight of 12.15 g/mole of electrons and a high melting point of 649° C. Compared to lithium, it is easy to handle, machine and dispose. Because of its greater relative abundance, it is lower in cost as a raw material than lithium and magnesium compounds are generally of lower toxicity than lithium compounds. All of these properties coupled with magnesium's reduced sensitivity to air and moisture compared to lithium, combine to make magnesium an attractive alternative to lithium as an anode material.
Magnesium (Mg) batteries are being researched as a candidate for post lithium-ion systems. They are expected to be high energy battery systems, due to the high volumetric capacity made available via the two electron transfer per Mg. However, a cathode active material compatible with magnesium and providing high capacity and durability is a subject of much ongoing investigation.
Examples of cathode active materials for magnesium electrochemical cells which are conventionally known include sulfur, MnO2 and a Chevrel compound having a formula MgxMo6Tn, wherein x is a number from 0 to 4, T is sulfur, selenium or tellurium, and n is 8.
The inventors have previously identified a K ion stabilized α-MnO2 as showing very high reversible capacity (U.S. 2013/0004830 A1).
It is conventionally known that MnO2 can assume various polymorphic phases depending on factors which include, for example, method of synthesis, thermal history and age. The polymorphic phases may be described in terms of the structural relationship of MnO6 octrahedra which may be linked at corners and edges. Three main groups of structures of the octahedral framework are known and are designated as: 3D tunnels, 2D layers and 1D channel.
Of these groups MnO2 with 3D tunnels is described as the spinel phase which can be synthesized via solid state reaction at elevated temperatures, typically temperatures of about 750° C.
The 2D layered MnO2 may be prepared by “low temperature” methods, such as by oxidation reaction in an alkaline solution or by a reduction of permanganates in an acid medium. Normally, this type of MnO2 is of poor crystallinity and contains a significant amount of water as well as stabilizing cations such as K+ between the sheets of MnO6 octrahedra.
The 1D channel framework MnO2 includes a broad range of polymorphic structures classified as α-, β-, δ- and γ-phases. Under certain circumstances these phases may be interchangeable depending on temperature and solution form.
However, the physical and chemical factors related to the various phases of MnO2 which affect the capacity of the material as a cathode active agent and how to optimize those physical factors to improve cathodic performance has not been described.
Yamamoto et al. (U.S. 2010/0196762) describes a manganese oxide obtained by reduction of potassium permanganate in hydrochloric acid, filtration and washing of the formed precipitate, then drying and heat treating at 300 to 400° C. There is no description of surface area of the particles or relationship of surface area to performance. Mg electrochemical cells are recited in Claims 14 to 16.
Shon et al. (U.S. 2013/0004850) describe an ordered porous manganese oxide of formula:MnOxOy wherein a ratio y/x is less than 2. Example compounds include MnO, Mn2O3, and Mn3O4. The porous manganese oxide is prepared by impregnating a template material such as SiO2 with a solution or melt of a Mn salt, sintering the impregnated material to obtain a composite and then chemically removing the template from the resulting porous manganese oxide. Sintering temperatures of 300 to 700° C. are suitable for the preparation. The specific surface area of the porous manganese oxide is from about 50 to 250 m2/g. Use of the porous manganese oxide as an active component of an electrode is described. Specifically, utility as a positive active material in lithium batteries or electrochemical cells and capacitors is disclosed.
Padhi et al. (U.S. 2011/0070487) describe manganese oxides having octahedral molecular sieve structure as active catalyst materials for a metal-air cell. Mn2O3 is disclosed as a component of the manganese oxide. The octahedral manganese oxide is obtained by redox reaction of a manganese salt such as manganese sulfate (MnSO4) or manganese nitrate (Mn(NO3)2) with a permanganate in aqueous acid, at a pH less than about 4.5 at a temperature of 50° C. to 70° C. The material obtained is described as cryptomelane and this synthesis product is dried and calcined at a temperature of from 95 to about 650° C. In preferred embodiments, calcination is conducted at temperatures of from 450 to 650° C. Construction of an electrochemical cell, i.e., a metal-air cell employing the calcined cryptomelane as cathode active ingredient is disclosed. The metal of the anode is zinc, lithium or aluminum.
Yamamoto et al. (U.S. 2009/0068568) describe a magnesium ion containing non-aqueous electrolyte for an electrochemical device such as a magnesium battery. The electrolyte is prepared by addition of magnesium metal to an ether solution mixture of a halogenated hydrocarbon, an aluminum halide and a quaternary ammonium salt. The mixture is heat treated to obtain the electrolyte. A magnesium battery containing the electrolyte is disclosed wherein a positive electrode containing an oxide or halide of a metal such as scandium, copper, chromium and manganese among others is described.
Xu et al. (U.S. 2005/0135993) describe an amorphous nanostructured cation-doped manganese oxide which is prepared by reduction of an aqueous permanganate solution to form a hydrogel of manganese oxide containing a dopant cation. The hydrogel is cryogenically frozen and vacuum dried to obtain a product having a BET surface area of greater than 300 m2/g. High performance is attributed to the high surface area nano-architecture of the manganese oxide. Electrochemical cycling performance with nonaqueous Li solution is described as well as utility as a reversible intercalation host for lithium in a cathode of a rechargeable battery.
Nazri (U.S. Pat. No. 5,604,057) describes a sub-micron size amorphorous, microporous manganese oxide which has an internal surface area greater than about 100 m2/g. The process to make this material includes reduction of permanganate with a manganous salt at low pH and at concentrations less than 0.3 molar. Addition and reaction is conducted with rapid stirring to form a single phase gel containing quatravalent manganese oxide. The gel is dewatered and dried under vacuum at temperatures of 180° C. or less. Electrodes are prepared by mixing the dried particles with a binder, coating a conductive support with the mixture and drying. The formed electrode is described as lithium-intercalateable and the claims recite secondary lithium cell.
Davis et al. (U.S. Pat. No. 6,585,881) describe an electrolysis method for preparing manganese dioxide having a specific surface area from 18 to 45 m2/g. A doping agent such as a soluble titanium dopant is included to obtain a surface area in the stated range. Utility as a cathode material for a primary alkaline cell is described. This reference actually teaches away from a specific surface area above 45 m2/g.
Feddrix et al. (U.S. Pat. No. 7,501,208) describes a doped manganese dioxide electrode material made electrolytically (EMD) or by a wet chemical method (CMD). The manganese dioxide described is preferably a γ-MnO2.
None of these references discloses or suggests a relationship of surface area of a phase of MnO2 to cathodic performance in a magnesium cell.
The inventors are directing effort and resources to the study of cathode materials useful to produce a magnesium battery of sufficient capacity and cycle lifetime to be useful as a power source for utilities requiring a high capacity and high cycle lifetime. Particularly, the inventors are investigating the chemical and physical properties of MnO2 and the relationship of those properties to performance as an active cathode material in a magnesium cell or battery, preferably a rechargeable magnesium battery.
Therefore, an object of the present invention is to provide a magnesium cell containing an active cathode material which is suitable for utility as a battery having high capacity and high cycle lifetime.
A second object of the invention is to provide a rechargeable magnesium battery having high capacity and high cycle lifetime.